Many of the recent automatic transmissions for automobiles are of a construction wherein the states of engagement of plural frictional engaging elements, including hydraulic clutches and brakes, are switched from one to another state by operation of a hydraulic control circuit, whereby plural shift ranges are attained. In an automatic transmission of such a construction, when a sufficient engine brake force is not obtained even if an accelerator is turned OFF for example on a downhill, a driver turns OFF an overdrive switch or switches a shift lever from D Range to S or L Range to effect a down-shift, thereby increasing the engine brake force.
When a down-shift for increasing the engine brake force is performed on the basis of a driver's intention of deceleration (e.g., decelerating operation) and with the accelerator OFF, the transmission gear ratio of the automatic transmission becomes larger as a result of the down-shift and therefore it is necessary to so much increase the rotational speed of the engine. However, in an operation mode requiring such an engine brake, the throttle valve is usually closed, so that an output-side torque is transmitted to the engine side by the transfer of torque through frictional engaging elements which are for attaining a shift range after the down-shift, whereby the rotational speed of the engine is increased. Consequently, the time required for the shift becomes longer and the effect of engine brake may not be obtained at a required timing, or an inertia torque induced with an increase in the rotational speed of the engine appears as a braking torque for the vehicle, with a consequent temporary increase of the engine brake force and generation of a shift shock. Moreover, if the transfer torque of the frictional engaging elements is suddenly increased for example by a hydraulic control of the automatic transmission, the rotational speed of the engine increases quickly and hence the shift time becomes shorter, but the braking torque increases rapidly, resulting in the shift shock becoming more marked.
With a view to solving such problems, a control technique is proposed in Japanese Patent No. 2924463. This control technique uses engine output increasing means for increasing an engine output temporarily when an automatic transmission is shifted down to a lower speed range in which engine brake acts with an accelerator substantially OFF and a timer for measuring an elapsed time from a predetermined measurement start point such as, for example, a shift output point at which a hydraulic control circuit is switched from one to another state at the time of down-shift. According to this proposed control technique, an engine output increasing control by the engine output increasing means is started on the basis of the elapsed time measured by the timer so that the rotational speed of the engine increases after a high speed range-side frictional engaging element which is released at the time of down-shift begins to slip and until complete engagement of a low speed range frictional element which is brought into engagement at the time of down-shift. The control start timing is set on the basis of a vehicular operating condition (more specifically, the temperature of oil in the hydraulic control circuit or the rotational speed of the engine) which exerts an influence on at least one of engagement/release delay times of the frictional engaging elements and an engine output increase delay time.
In the Japanese patent No. 2924463 it is also disclosed that there is a delay time until actual release or engagement of a frictional engaging element in an automatic transmission and there also is a delay time after a throttle angle control for increasing the engine output has been performed and until actual increase of the engine output and that therefore, by setting the start timing taking those delay times into account, the shift time is shortened while suppressing the occurrence of a shift shock. Also, according to the disclosure of the patent in question, it is preferable to control the degree of opening of a throttle valve so that the engine speed increases in conformity with a slip start timing of the frictional engaging element on the high speed range side.
In JP-7-247874A, with a view to solving the foregoing problems, there is disclosed a technique in which, when an automatic transmission is shifted down by manual operation to a low speed range undergoing the action of engine brake with an accelerator substantially OFF, the amount of intake air is increased and restoring (fuel injection) from fuel cut is performed to increase the engine output. According to this technique disclosed in the '874 publication, an ISC valve for idling control is opened to increase the amount of intake air before the shift is started in the above shift-down, and when the start of the shift is detected by a change in rotation of a rotating member, the supply of fuel is resumed by a fuel cut restoring control to increase the engine torque, thereby shortening the shift time and preventing the occurrence of a shift shock. In JP-10-18877A there is proposed a technique in which, taking note of a shift shock diminishing effect by an engine output increasing control in down-shift, a torque increase quantity by the engine output increasing control is made larger in an automatic down-shift according to a preset shift schedule than in manual down-shift. According to this proposed technique, in manual down-shift, the torque increase quantity is made relatively small for generating a moderate shift shock which affords a deceleration feeling, while in an automatic down-shift involving execution of a shift independently of operation performed by a driver, the torque increase quantity is made relatively large so as not to let the driver feel a shift shock.
However, the delay time until actual release or engagement of the associated frictional engaging element referred to above at the time of down-shift varies depending on not only the temperature of oil in the hydraulic control circuit or the engine speed but also the vehicle speed or the torque acting on the frictional engaging element when the down-shift control is performed. Particularly, at the time of down-shift, since the accelerator pedal is substantially fully closed, it is necessary to take into account that an arbitrary drive torque below the road load (below the torque required for constant speed running at the speed of the time point concerned) is applied from the engine side. The delay time is also influenced by an operating condition including a slip control for a lock-up clutch being executed. Therefore, in a timer-based setting of a control start time, the throttle valve position control (engine output increasing control) cannot always be started at an appropriate timing even if the influence of the oil temperature or the engine speed is taken into account. Thus, there is a fear that the start timing of the throttle valve position control may be offset from the appropriate timing and the driver may be given an acceleration or a shock by the throttle valve position control during down-shift. Besides, for appropriately setting a reference value of the timer taking the influence of the oil temperature and engine speed into account, not only it is necessary in a conforming process to set a reference value based on repeated experiments but also it is necessary to again set the timer reference value in the case where there arises the necessity of changing the oil pressure removing method due to a change of specification for the hydraulic shift control. Thus, not only the logic becomes complicated but also it is necessary to set many parameters and the parameter conforming work is very troublesome.
Moreover, even if the engine output increasing control is started in a proper manner, an unpleasant shock will be induced if a termination timing of the control is not appropriate. For example, in the foregoing down-shift, if an engagement-side frictional engaging element has a sufficient transfer torque capacity and if the engine output increasing control is continued in this state even after the down-shift reaches a nearly terminated state, an increased engine output causes the vehicle to be accelerated by a gear on the low speed range side, so that an unpleasant shock as a push-out feeling is developed. Conversely, if the engine output increasing control is terminated in a state in which the engagement-side frictional engaging element does not yet have a transfer torque capacity sufficient for terminating the down-shift, the frictional engaging element acts in a direction in which the progress of the down-shift is decelerated, so that not only the shift time is not shortened, but also a strong deceleration shock is developed by both a decrease of the engine torque resulting from termination of the control and a coast torque applied from the vehicle side.
Further, it is necessary that the engine output quantity increased in the down-shift be controlled to a proper quantity conforming to conditions. More particularly, in the case where the output increase quantity is excess, not only an excess increase of the engine speed occurs and an extra time is taken until the end of shift, but also, for decreasing the increased engine speed by the engaging force of the engagement-side frictional engaging element, a thermal load on the frictional engaging element increases and there occurs a shift shock based on inertia energy. On the other hand, in the case where the output increase quantity is deficient, there arises the same condition as in the conventional shift control for increasing the engine speed by the engagement-side frictional engaging element and therefore the inertia torque consumed for increasing the engine speed cannot sufficiently mitigate the shift shock which is for becoming a vehicle braking torque. Thus, it is necessary that the output increase quantity in the engine output increasing control in the down-shift be set and controlled so as to afford a desired engine speed change quantity in the down-shift.
Since the engine output increasing control is performed by an increase of the intake air quantity and the supply of fuel proportional thereto, not depending on a driver's operation, the application of the same control also to a down-shift based on a predetermined shift schedule taking into account a slow deceleration such as that in street running or running on a congested road affords the shift shock diminishing effect disclosed in JP-10-18877A, but causes the generation of noise due to a sudden increase of the engine speed and the deterioration of fuel economy due to an increase of fuel consumption resulting from this control, and is therefore not advisable.